Fast fourier transform signal processing method for doppler radar speed sensor

Abstract

A vehicle speed sensing system includes an RF transceiver coupled to an antenna for transmitting an RF signal towards the terrain over which the vehicle moves and for receiving a reflected Doppler signal therefrom. The transceiver generates a time-domain in-phase reference signal I and a time-domain quadrature signal Q which is offset in phase by 90 degrees from the reference signal I. A digital signal processor which receives the I and Q signals, and uses a complex fast Fourier transform routine to convert the time domain I and Q signals to frequency domain values I(f) and Q(f). The digital signal processor further processes the I(f) and Q(f) values and generates a speed a direction signal which is unaffected by vehicle vibrations.

We claim:  
     
       1. A signal processing method for a vehicle speed sensing system having an RF transceiver coupled to an antenna for transmitting an RF signal towards the terrain over which the vehicle moves and for receiving a reflected Doppler signal therefrom, the transceiver generating a time-domain in-phase reference signal I and a time-domain quadrature signal Q which is offset in phase by 90 degrees from the reference signal I, and a digital signal processor which receives the I and Q signals, the signal processing method comprising: 
       executing a complex fast Fourier transform routine to convert the time domain I and Q signals to frequency domain values I(f) and Q(f);  
       converting the frequency domain values I(f) and Q(f) to a vehicle speed signal;  
       identifying vibration domain frequency values resulting from vehicle vibration; and  
       preventing such vibration frequency domain values from effecting the conversion of the frequency domain values I(f) and Q(f) to the vehicle speed signal.  
     
     
       2. The signal processing method of  claim 1 , further comprising: 
       obtaining and storing reverse and forward frequency domain values with corresponding amplitude values, each forward frequency domain value representing a forward vehicle speed, each reverse frequency domain value representing a reverse vehicle speed, and each reverse domain frequency value having a corresponding mirror image forward frequency domain value representing an equal and opposite vehicle speed; and  
       calculating the ratio of an amplitude of a frequency domain value to an amplitude of a corresponding mirror image frequency domain value; and  
       if said ratio is less than a threshold, preventing such frequency domain value from being converted to the vehicle speed value.  
     
     
       3. The signal processing method of  claim 2 , wherein: 
       each reverse frequency domain value represents a speed equal to and opposite from its corresponding mirror image forward frequency domain value.